Java Reference
In-Depth Information
Code 7.6
A state-reporting
* Print the values of this object's fields.
* @param where Where this state occurs.
public void reportState(String where)
System.out.println( "displayValue: " + displayValue +
" leftOperand: " + leftOperand +
" previousOperator: " +
previousOperator + " at " + where);
If each method of CalcEngine contained a print statement at its entry point and a call to
reportState at its end, Figure 7.8 shows the output that might result from a call to the tester's
testPlus method. (This was generated from a version of the calculator engine that can be
found in the calculator-engine-print project.) Such output allows us to build up a picture of how
control flows between different methods. For instance, we can see from the order in which the
state values are reported that a call to plus contains a nested call to applyPreviousOperator .
Figure 7.8
Debugging output
from a call to
clear called
displayValue: 0 leftOperand: 0 previousOperator: at end of clear
numberPressed called with: 3
displayValue: 3 leftOperand: 0 previousOperator: at end of number...
plus called
applyPreviousOperator called
displayValue: 3 leftOperand: 3 previousOperator: at end of apply...
displayValue: 0 leftOperand: 3 previousOperator: + at end of plus
numberPressed called with: 4
displayValue: 4 leftOperand: 3 previousOperator: + at end of number...
equals called
displayValue: 7 leftOperand: 0 previousOperator: + at end of equals
Print statements can be very effective in helping us understand programs or locate errors, but
there are a number of disadvantages:
It is not usually practical to add print statements to every method in a class. So they are only
fully effective if the right methods have been annotated.
Adding too many print statements can lead to information overload. A large amount of out-
put can make it difficult to identify what you need to see. Print statements inside loops are a
particular source of this problem.
Once their purpose has been served, it can be tedious to remove them.
There is also the chance that, having removed them, they will be needed again later. It can be
very frustrating to have to put them back in again!
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